(a) Technical Field
The present invention relates to a fuel cell and a method for manufacturing the same. More particularly, it relates to a fuel cell with enhanced mass transfer characteristics and a method for manufacturing the same, which can reduce water flooding by efficiently discharging water produced by an electrochemical reaction using a highly hydrophobic gas diffusion layer having a new surface structure and can improve cell performance in a high power density region and in an abnormal operating condition by facilitating the supply of reactant gases such as hydrogen and air (oxygen) to a membrane-electrode assembly.
(b) Background Art
Typically, one of the most attractive fuel cells for a vehicle is a polymer electrolyte membrane fuel cell (PEMFC) manufactured by stacking several hundreds of unit cells into a stack. In order to mount the PEMFC in a vehicle for use of transport, the PEMFC should exhibit a high power performance of several tens of kW or higher under various operating conditions and, to this end, should be able to stably operate in a wide current density range.
The electrochemical reaction for electricity generation of the PEMFC will be described below. Hydrogen supplied to an anode as an oxidation electrode in a membrane electrode assembly (MEA) of the fuel cell is dissociated into hydrogen ions (protons) and electrons. The hydrogen ions are transmitted to a cathode as a reduction electrode through a polymer electrolyte membrane, and the electrons are transmitted to the cathode through an external circuit so that electricity is generated by the flow of electrons. Moreover, at the cathode, the protons, electrons and oxygen molecules react with each other to produce electricity and heat and, at the same time, produce water as a reaction by-product.
The area expressing the electrochemical performance of the fuel cell is generally classified into three regions: (i) an “activation loss” region due to loss of electrochemical reaction kinetics; (ii) an “ohmic loss” region due to contact resistance at interfaces between respective components and loss of ionic conduction in the polymer electrolyte membrane; and (iii) a “mass transport or transfer loss” or “concentration loss” region due to the limitations of mass transport of reactant gases [See R. O Hayre, S. Cha, W. Colella, F. B. Prinz, Fuel Cell Fundamentals, Ch. 1, John Wiley & Sons, New York (2006), which is hereby incorporated by reference].
When an appropriate amount of water produced during the electrochemical reaction is present, it preferably serves to maintain the humidity of the polymer electrolyte membrane. However, when an excessive amount of water produced is not appropriately removed, “flooding” occurs at a high current density, preventing the reactant gases from being efficiently supplied to the fuel cell and thereby increasing voltage loss [See M. M. Saleh, T. Okajima, M. Hayase, F. Kitamura, T. Ohsaka, J. Power Sources, 167, 503 (2007), which is hereby incorporated by reference].
Recently, with the commercialization of the fuel cell, extensive research and development of gas diffusion layers as a key component of water management in the fuel cell has continued to progress. A gas diffusion layer which is typically included in the fuel cell will be described in detail below.
A typical porous medium that constitutes the fuel cell is a gas diffusion layer (GDL), which is composed of both a microporous layer (MPL) and a macroporous substrate or backing.
At present, commercially available gas diffusion layers have a dual layer structure including a microporous layer (MPL) having a pore size below 1 micrometer when measured by mercury intrusion and a macroporous substrate or backing having a pore size of 1 to 300 micrometers [See, X. L. Wang, H. M. Zhang, J. L. Zhang, H. F. Xu, Z. Q. Tian, J. Chen, H. X. Zhong, Y. M. Liang, and B. L. Yi, Electrochimica Acta, 51, 4909 (2006) which is hereby incorporated by reference].
The gas diffusion layer is attached to the outer surface of each of catalyst layers for the anode and cathode coated on both surfaces of the polymer electrolyte membrane in the fuel cell. The gas diffusion layer functions to supply reactant gases such as hydrogen and air (oxygen), transmit electrons produced by the electrochemical reaction, and discharge water produced by the reaction to minimize the flooding phenomenon in the fuel cell [See L. Cindrella, A. M. Kannan, J. F. Lin, K. Saminathan, Y. Ho, C. W. Lin, J. Wertz, J. Power Sources, 194, 146 (2009); and X. L. Wang, H. M. Zhang, J. L. Zhang, H. F. Xu, Z. Q. Tian, J. Chen, H. X. Zhong, Y. M. Liang, B. L. Yi, Electrochim. Acta, 51, 4909 (2006) which are both hereby incorporated by reference].
Typically, the microporous layer of the gas diffusion layer may be formed by preparing a mixture of carbon black powder such as acetylene black carbon, black pearl carbon, etc. and a hydrophobic agent such as polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP). The mixture can be coated on one or both sides of the macroporous substrate.
The macroporous substrate of the gas diffusion layer is generally composed of carbon fiber and a hydrophobic agent such as polytetrafluoroethylene and fluorinated ethylene propylene [See, C. Lim and C. Y. Wang, Electrochim. Acta, 49, 4149 (2004)] and may be broadly classified into carbon fiber felt, carbon fiber paper, and carbon fiber cloth [S. Escribano, J. Blachot, J. Etheve, A. Morin, R. Mosdale, J. Power Sources, 156, 8 (2006); M. F. Mathias, J. Roth, J. Fleming, and W. Lehnert, Handbook of Fuel Cells-Fundamentals, Technology and Applications, Vol. 3, Ch. 42, John Wiley & Sons (2003) which are both hereby incorporated by reference].
It is necessary to optimize the structural design of the gas diffusion layer for the fuel cell such that the gas diffusion layer provides appropriate performance according to its application fields, such as transportation, portable, and residential power generation devices, and the fuel cell operational conditions. In general, in the formation of the gas diffusion layer for a fuel cell vehicle, the carbon fiber felt or carbon fiber paper is preferred to the carbon fiber cloth since the carbon fiber felt and carbon fiber paper have excellent properties such as reactant gas supply properties, product water discharge properties, compression properties, and handling properties.
Moreover, the gas diffusion layer has a significant effect on the performance of the fuel cell according to complex and various structural differences such as the thickness, gas permeability, compressibility, hydrophobicity of microporous layer and macroporous substrate, carbon fiber structure, porosity/pore size distribution, pore tortuosity, electrical resistance, bending stiffness, etc. Especially, it is known that there is a significant difference in performance in the mass transport region [See D. H. Ahmed, H. J. Sung, and J. Bae, Int. J. Hydrogen Energy, 33, 3767 (2008); and Y. Wang, C. Y. Wang, and K. S. Chen, Electrochim. Acta, 52, 3965 (2007); and C. J. Bapat and S. T. Thynell, J. Power Sources, 185, 428 (2008); which are hereby incorporated by reference].
In particular, in order to increase the mass transfer characteristics and maintain high cell performance by effectively removing the water produced during the electrochemical reaction of the fuel cell, it is very important to impart hydrophobicity to the microporous layer and the macroporous substrate by appropriately introducing a hydrophobic agent such as polytetrafluoroethylene (PTFE) into them [See S. Park, J.-W. Lee, B. N. Popov, J. Power Sources, 177, 457 (2008); and G.-G. Park. Y.-J. Sohn, T.-H. Yang, Y.-G. Yoon, W.-Y. Lee, C.-S. Kim, J. Power Sources, 131, 182 (2004) which are hereby incorporated by reference].
However, a wet chemical process has conventionally been used to impart hydrophobicity, and thus the manufacturing process itself is complicated and it is difficult to uniformly distribute the hydrophobic agent such as PTTE on the gas diffusion layer.
Moreover, according to the conventional process for manufacturing the gas diffusion layer, it is difficult to further impart high hydrophobicity or super-hydrophobicity corresponding to a contact angle (static constant angle) of 150° or higher to a porous medium which have already been subjected to waterproof treatment.
In conventional studies, there are various attempts to impart hydrophilicity to the surface of the porous medium using various plasma processes such as oxygen, nitrogen, ammonia, silane (SiH4), organometallics, etc., which, however, are different from the object of the present invention to impart high hydrophobicity to the porous medium.
In addition, there are attempts to employ plasma surface treatment techniques during the formation of the electrodes of the MEA, which, however, relate to a process for forming a catalyst layer comprising catalyst and binder. That is, these methods are to chemically form a hydrophilic or hydrophobic surface by modifying the surface of the catalyst layer using plasma techniques, and with these methods, it is very difficult to form high hydrophobicity on the surface of the porous medium.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.